Finely chopped and pound the white parts of lemon grass, add in the
other ingredients of marinade (except fish sauce and brown sugar), make a course paste them using a mortar and pestle, scoop out and mix with fish sauce and brown sugar.

Rinse the chicken and drain well. Coat them with marinade, for 1 hour or overnight in fridge.

Heat oil in a grill pan, remove chicken from the fridge, discard the
marinade. Place the chicken fillet on the pan and grill till both sides
are cooked and lightly brown.

In another pan, heat some oil and saute garlic till aromatic. Pour
in the other ingredients for sauce, bring it to boil. Add in corn flour +
water as thickening agent and cook to preferred consistency.